Power switch reverse current protection systems

ABSTRACT

One example described herein includes a power switch control system. The system includes a first monitoring terminal coupled to a first terminal of a power transistor and a second monitoring terminal coupled to a second terminal of the power transistor. The power transistor and the power switch control system can form an ideal diode between the first monitoring terminal arranged as an anode and the second monitoring terminal arranged as a cathode. The system further includes a reverse current controller coupled to the first monitoring terminal and the second monitoring terminal and is configured to control activation of the power transistor to conduct a reverse current from the second monitoring terminal to the first monitoring terminal in response to a reverse voltage arranged as a cathode voltage at the second monitoring terminal being greater than an anode voltage at the first monitoring terminal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Indian Provisional PatentApplication No. 202141021460, filed 12 May 2021, and from IndianProvisional Patent Application No. 202141014731, filed 31 Mar. 2021,which are both incorporated herein by reference in their entirety.

TECHNICAL FIELD

This description relates generally to electronic circuits, and moreparticularly to power switch reverse current protection systems.

BACKGROUND

Power supply systems that implement a power transistor to delivercurrent to a load are prevalent in a large variety of applications. Someapplications implement a large transistor device to providecorrespondingly large current to a load. One such application is forautomotive electronic systems. In such power supply systems, the load istypically provided in an output stage that includes some reactivecircuit components, such as an inductor. In response to certainconditions, the reactive components can provide a large reverse voltageacross the power transistor, which can be exhibited as a large reversecurrent. The reverse current can be potentially destructive to the powertransistor and/or other circuit devices in the power supply system. Somepower supply systems can include devices and subcircuits that canprovide some degree of protection or dissipation of the reverse current,such as a transient voltage suppressor (TVS) arranged in an input stageof the power supply system.

SUMMARY

One example described herein includes a power switch control system. Thesystem includes a first monitoring terminal coupled to a first terminalof a power transistor and a second monitoring terminal coupled to asecond terminal of the power transistor. The power transistor and thepower switch control system can form an ideal diode between the firstmonitoring terminal arranged as an anode and the second monitoringterminal arranged as a cathode. The system further includes a reversecurrent controller coupled to the first monitoring terminal and thesecond monitoring terminal and is configured to control activation ofthe power transistor to conduct a reverse current from the secondmonitoring terminal to the first monitoring terminal in response to areverse voltage arranged as a cathode voltage at the second monitoringterminal being greater than an anode voltage at the first monitoringterminal.

Another example described herein includes a power supply system. Thesystem includes an input stage configured to provide a power current inresponse to an input voltage and an output stage configured to providethe power current to a load. The system also includes a power transistorarranged between the input stage and the output stage. The powertransistor can be activated to conduct the power current from the inputstage to the output stage. The system further includes a power switchcontrol system. The power switch control system includes a firstmonitoring terminal coupled to a first terminal of a power transistorand a second monitoring terminal coupled to a second terminal of thepower transistor. The power transistor and the power switch controlsystem can form an ideal diode between the first monitoring terminalarranged as an anode and the second monitoring terminal arranged as acathode. The power switch control system further includes a reversecurrent controller coupled to the first monitoring terminal and thesecond monitoring terminal and is configured to control activation ofthe power transistor to conduct a reverse current from the secondmonitoring terminal to the first monitoring terminal in response to areverse voltage arranged as a cathode voltage at the second monitoringterminal being greater than an anode voltage at the first monitoringterminal

Another example described herein includes an integrated circuit. Thecircuit includes a Zener diode stack having an input coupled to a firstmonitoring terminal and an output coupled to a second monitoringterminal. The first and second monitoring terminals can be coupled toreceive a first terminal of a power transistor and a second terminal ofthe power transistor, respectively. The circuit also includes an anoderegulation transistor device having an input coupled to the Zener diodestack, a first terminal coupled to the first monitoring terminal, and asecond terminal coupled to receive an input of the power transistor. Thecircuit further includes a cathode regulation transistor device havingan input coupled to the Zener diode stack, a first terminal coupled toreceive the input of the power transistor, and a second terminal coupledto the second monitoring terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example block diagram of a power supply system.

FIG. 2 is an example diagram of a power switch controller circuit.

FIG. 3 is another example diagram of a power switch controller circuit.

DETAILED DESCRIPTION

This description relates generally to electronic circuits, and moreparticularly to power switch reverse current protection systems. A powersupply system can include a power switch control system that can becoupled to a gate of a power transistor, so the power switch controlsystem can control operation of the power transistor. The power switchcontrol system can include a reverse current controller that can provideprotection against damage to the power transistor and/or other circuitcomponents resulting from a large reverse current that can be providedthrough the power transistor. For example, the large reverse current canoccur based on deactivation of the power transistor to cease providingcurrent through an inductor in an output stage, thus resulting in alarge and rapidly provided reverse current. As an example, the reversecurrent event can be modeled based on a number of known industrystandards, such as the automotive standard ISO 7637 in which a largereverse voltage (e.g., approximately 150 volts) can appear across thepower transistor.

As described herein, the reverse current controller can operate to clampa reverse voltage to limit the amplitude of the reverse current throughthe power transistor. The reverse current controller includes a firstmonitoring terminal and a second monitoring terminal that are coupled torespective terminals (e.g., source and drain, respectively) of the powertransistor, so the power transistor and the reverse current controllerform an ideal diode in which the first monitoring terminal behaves as ananode of the ideal diode and the second monitoring terminal behaves asthe cathode of the ideal diode. The reverse voltage can therefore be areverse voltage in which the cathode voltage at the second monitoringterminal that is greater than an anode voltage at the first monitoringterminal.

The reverse current controller includes a Zener diode stack that isconfigured to set a particular threshold amplitude for the reversevoltage, and further includes a cathode regulation transistor device andan anode regulation transistor device that each have inputs coupled tothe Zener diode stack. The input (e.g., gate) of the power transistor isarranged between the cathode regulation transistor device and the anoderegulation transistor device. Therefore, in response to the reversevoltage being approximately equal to the threshold amplitude, thecathode regulation transistor device and the anode regulation transistordevice can each activate to activate the power transistor, therebyconducting the reverse current from the cathode to the anode. Becausethe input of the power transistor is arranged between the cathoderegulation transistor device, changes to either the cathode voltage orthe anode voltage result in changes to activation of the powertransistor to regulate the reverse current through the power transistor,thereby providing clamping of the reverse voltage. Accordingly, theclamping of the reverse voltage can limit the amplitude of the reversecurrent, thereby protecting the power transistor and/or the othercircuit components of the power supply system. Such a clamping of thereverse voltage, and thus limiting of the reverse current, can obviatethe need for other bulky current protection devices, such as a transientvoltage suppressor (TVS).

As described herein, the term “activate”, as describing a transistordevice, refers to providing sufficient bias (e.g., gate-source voltagefor a field-effect transistor (FET)) to operate the transistor device insaturation mode. Similarly, the term “deactivate”, as describing atransistor device, refers to removing bias to operate the transistordevice in cutoff mode.

FIG. 1 is an example block diagram of a power supply system 100. Thepower supply system 100 can be implemented in any of a variety of powerproviding applications, such as in an automotive electronics system. Asdescribed herein, the power supply system 100 can be effective inmitigating potentially destructive large reverse currents.

The power supply system 100 includes an input stage 102 that isconfigured to generate a power current I_(PW) based on an input voltageV_(IN). The power supply system 100 also includes an output stage 104that is configured to provide the power current I_(PW) to a load (notshown) via a power transistor 106, demonstrated in the example of FIG. 1as an N-channel field-effect transistor (FET) having a source coupled toa terminal 108 that is likewise coupled to the input stage 102, andincludes a drain coupled to a terminal 110 that is likewise coupled tothe output stage 104. The power supply system 100 further includes apower switch control system 112 that is coupled to a gate of the powertransistor 106, and is therefore configured to control operation of thepower transistor 106. As an example, the power switch control system 112can be provided in or as part of an integrated circuit (IC). Asdescribed herein, the power switch control system 112 and the powertransistor 106 can be arranged to as an ideal diode, so the terminal 108can be an anode of the ideal diode and the terminal 110 can be a cathodeof the ideal diode. The terminal 108, and thus the anode, isdemonstrated as having a voltage V_(A). Similarly, the terminal 110, andthus the cathode, is demonstrated as having a voltage V_(C).

In the example of FIG. 1, the power switch control system 112 includes areverse current controller 114. The reverse current controller 114 iscoupled to the terminal 108 and the terminal 110, so the terminals 108and 110 can be respective first and second monitoring terminals fordetecting a reverse voltage V_(CA) across the power transistor 106. Forexample, during normal operating conditions, the power transistor 106 isperiodically activated by the reverse current controller 114 to providethe power current I_(PW) from the input stage 102 to the output stage104, so the voltage V_(A) is greater than the voltage V_(C). However, incertain conditions (e.g., cessation of the power current I_(PW) to aninductor in the output stage 104), the voltage V_(C) can increaserapidly relative to the voltage V_(A), causing a non-zero reversevoltage V_(CA) to develop across the power transistor 106. As anexample, the reverse voltage V_(CA) can be very large (e.g.,approximately 150 volts), such as modeled by an industry standard test(e.g., ISO 7637 for automotive industry testing). Therefore, a resultingreverse current, demonstrated as a current I_(REV) in the example ofFIG. 1, can flow from the output stage 104 to the input stage 102, andthus from the cathode to the anode, which can potentially damage thepower transistor 106 and/or other circuit components in the input stage102.

To mitigate damage to the power transistor 106 and/or other circuitcomponents in the input stage 102, the reverse current controller 114can be configured to clamp the voltage V_(CA) to a particular thresholdamplitude by activating the power transistor 106, thus decreasing thepotential amplitude of the reverse current I_(REV). As an example, thereverse current controller 114 can include a Zener diode stack that canset the threshold amplitude, and can include a pair of transistordevices that are coupled to the respective terminals 108 and 110 thateach provide contributions to the control of the power transistor 106.Therefore, in response to changes to an increase in the amplitude of thevoltage V_(C) or a decrease in the amplitude of the voltage V_(A), thepair of transistor devices can adjust the control of the powertransistor 106 to adjust the amplitude of the reverse current I_(REV) tomaintain clamping of the voltage V_(CA) at the threshold amplitude.Accordingly, the reverse current I_(REV) can be maintained at anamplitude that is sufficiently low to substantially mitigate damage tothe power transistor 106 and/or other circuit components in the inputstage 102.

Therefore, as described herein, the power supply system 100 can exhibitsufficient protection against reverse voltage conditions across thepower transistor 106 that can potentially result in damage to thecircuit. As described above, by clamping the voltage V_(CA) at thepredetermined threshold amplitude in response to a reverse voltagecondition, the reverse current I_(REV) can be maintained at an amplitudethat is sufficiently low to substantially mitigate damage to the powertransistor 106 and/or other circuit components in the input stage 102.By shifting the reverse current protection to the reverse currentcontroller 114 for providing activation and control of the powertransistor 106 to conduct the reverse current I_(REV), the power supplysystem 100 can obviate additional reverse current protection devices.For example, typical power supply systems can include a transientvoltage suppressor (TVS) in the input stage to conduct a reversecurrent. However, a TVS can be very large, and can thus occupy asignificant space on a circuit board (e.g., approximately 45% of thetotal area of the typical power supply system). Therefore, byimplementing the reverse current controller 114, the power supply system100 can be significantly more compact while providing sufficient reversecurrent protection.

FIG. 2 is an example block diagram 200 of a reverse current controller202. The diagram 200 also demonstrates a power transistor, demonstratedin the example of FIG. 2 as an N-FET N_(PWR). The reverse currentcontroller 202 and the power transistor N_(PWR) can be the reversecurrent controller 114 and the power transistor 106, respectively, inthe example of FIG. 1. Therefore, reference is to be made to the exampleof FIG. 1 in the following description of the example of FIG. 2.

In the example of FIG. 2, the reverse current controller 202 is coupledto a first monitoring terminal 204 and a second monitoring terminal 206.The power transistor N_(PWR) is arranged to have a source coupled thefirst monitoring terminal 204 and a drain coupled to the secondmonitoring terminal 206. As described above in the example of FIG. 1,the first and second monitoring terminals 204 and 206 can correspond,respectively, to an anode and a cathode of an ideal diode formed by thepower transistor N_(PWR) and the power switch control system 112.Therefore, the first monitoring terminal 204 can have an anode voltageV_(A) and the second monitoring terminal 206 can have a cathode voltageV_(C), and the power transistor N_(PWR) can have a reverse voltageV_(CA) across the drain-source, and thus across the cathode to anode.

In the example of FIG. 2, a control signal CTRL is provided to a gate ofthe power transistor N_(PWR). The control signal CTRL is representativeof an activation signal provided from control circuitry (not shown) thatis part of the power switch control system 112 to control a voltageV_(G) at the gate of the power transistor N_(PWR) for operating thepower transistor N_(PWR) during normal operation of the power supplysystem 100. As described herein, the control circuitry is disabled inresponse to a positive amplitude of the reverse voltage V_(CA), andcontrol of the power transistor N_(PWR) is provided solely by thereverse current controller 202. For example, the control circuitry caninclude a monitoring circuit that can monitor the amplitude of thevoltages V_(A) and V_(C), and thus the reverse voltage V_(CA), and candisable the control signal CTRL in response to a positive amplitude ofthe reverse voltage V_(CA).

In the example of FIG. 2, the reverse current controller 202 includes aresistor R₁ coupled to the second monitoring terminal 206 and a Zenerdiode stack 208 that is arranged between the first monitoring terminal204 and the resistor R₁. As an example, the Zener diode stack 208,combined with the resistor R₁, includes a quantity of Zener diodes thatcan set a desired predetermined threshold amplitude V_(PT) to which thereverse voltage V_(CA) is clamped. The reverse current controller 202also includes a cathode regulation transistor device, demonstrated as aP-channel FET P_(CR), having a source coupled to the second monitoringterminal 206 and a gate coupled between the resistor R₁ and the Zenerdiode stack 208. The drain of the cathode regulation transistor deviceP_(CR) is coupled to the gate via a Miller capacitor C_(M), and iscoupled to a gate of the power transistor N_(PWR) via a diode D₁. As anexample, the Miller capacitor C_(M) provides compensation for variationin the capacitance C_(GS), transconductance, and gain of the powertransistor N_(PWR) given that the power transistor N_(PWR) can beprovided as external to the IC that includes the power switch controlsystem 112. The reverse current controller 202 also includes an anoderegulation transistor device, demonstrated as an N-FET N_(AR), having asource coupled to the first monitoring terminal 204 and a gate coupledbetween two of the Zener diodes in the Zener diode stack 208 (e.g., afirst and a second Zener diode from the first monitoring terminal 204).The drain of the anode regulation transistor device N_(AR) is coupled tothe gate of the power transistor N_(PWR) via a resistor R₂.

During normal operation of the power supply system 100, the powertransistor N_(PWR) is controlled by the control signal CTRL (e.g., fromcontrol circuitry (not shown) in the power switch control system 112).During normal operation, current does not flow from the secondmonitoring terminal 206 to the first monitoring terminal 204 through theZener diode stack 208, thereby holding the anode regulation transistordevice N_(AR) and the cathode regulation transistor device P_(CR) in adeactivated state. Therefore, the power transistor N_(PWR) is controlledsolely by the control signal CTRL. In response to a reverse voltagecondition, when the cathode voltage V_(C) is greater than the anodevoltage V_(A), the control signal CTRL is disabled from controlling thepower transistor N_(PWR), thereby ceding control of the power transistorN_(PWR) to the reverse current controller 202. At an amplitude of thereverse voltage V_(CA) that is greater than zero but less than thepredetermined threshold amplitude VPT, the reverse voltage V_(CA) isinsufficient to overcome the breakdown voltage of the Zener diode stack208. Therefore, no current flows from the second monitoring terminal 206to the first monitoring terminal 204 through the Zener diode stack 208.

In response to the reverse voltage V_(CA) being approximately equal tothe predetermined threshold amplitude V_(PT), and thus approximatelyequal to the breakdown voltage of the Zener diode stack 208, a currentI_(Z) flows from the second monitoring terminal 206 to the firstmonitoring terminal 204 through the resistor R₁ and the Zener diodestack 208. The current I_(Z) thus provides a gate-source voltage foreach of the anode regulation transistor device N_(AR) and the cathoderegulation transistor device P_(CR), thus sufficiently activating eachof the anode regulation transistor device N_(AR) and the cathoderegulation transistor device P_(CR). The activation of the cathoderegulation transistor device P_(CR) provides a current I_(C) from thesecond monitoring terminal 206 through the cathode regulation transistordevice P_(CR) to the gate of the power transistor N_(PWR) via the diodeD₁, and the activation of the anode regulation transistor device N_(AR)provides a current I_(A) from the gate of the power transistor N_(PWR)through the resistor R₂ and through the anode regulation transistordevice P_(CR) to the first monitoring terminal 204. The amplitudes ofthe currents I_(A) and I_(C) can provide a sufficient amplitude of thevoltage V_(G) to hold the power transistor N_(PWR) in an activatedstate, thereby conducting the reverse current I_(REV) that flows fromthe second monitoring terminal 206 to the first monitoring terminal 204.

Nominally, the amplitude of the currents I_(A) and I_(C) can beapproximately equal. However, the amplitude of the currents I_(A) andI_(C) can be adjusted based on changes to the amplitude of therespective voltages V_(A) and V_(C). Thus, as described herein, theoperation of the cathode regulation transistor device P_(CR) and theanode regulation transistor device N_(AR) with respect to controllingthe gate of the power transistor N_(PWR) via the voltage V_(G) canoperate with negative feedback to maintain the reverse voltage V_(CA) atapproximately the predetermined threshold amplitude V_(PT). In theexample of FIG. 2, a voltage source 210 is demonstrated as coupled tothe first monitoring terminal 204, so a voltage V_(ISO) is provided tothe first monitoring terminal 204 through a resistor R_(ISO). Thevoltage V_(ISO) is intended to demonstrate variations that can occur tothe voltage V_(CA) (e.g., a change to the voltage V_(A) or to thevoltage V_(C)) during a reverse voltage condition (e.g., while the powertransistor N_(PWR) is activated to conduct the reverse current I_(REV)),such as resulting from active circuit components in the input stage 102or the output stage 104 of the power supply system 100.

An increase in the amplitude of the voltage V_(A) (as modeled by anincrease in the voltage V_(ISO)) during a reverse voltage condition, andthus a decrease in the amplitude of the reverse voltage V_(CA), resultsin the gate-source voltage V_(GS) of the cathode regulation transistordevice P_(CR) decreasing based on a change in the voltage across theresistor R₁. As a result of the decrease of the gate-source voltageV_(GS) of the cathode regulation transistor device P_(CR), theactivation of the cathode regulation transistor device P_(CR) (e.g., inthe saturation mode) changes to increase the amplitude of the currentI_(C) flowing through the cathode regulation transistor device P_(CR)and to the gate of the power transistor N_(PWR). In response, theincrease in the amplitude of the current I_(C) relative to the currentI_(A) results in an increase of the gate voltage V_(G) of the powertransistor N_(PWR). Accordingly, the activation of the power transistorN_(PWR) (e.g., in the saturation mode) changes to conduct a greateramplitude of the reverse current I_(REV), thereby increasing the reversevoltage V_(CA) to approximately the predetermined threshold amplitudeV_(PT), and thus the Zener breakdown voltage of the Zener diode stack208, in a reverse feedback manner.

Similarly, a decrease in the amplitude of the voltage V_(A) (as modeledby a decrease in the voltage V_(ISO)) during a reverse voltagecondition, and thus an increase in the amplitude of the reverse voltageV_(CA), results in the gate-source voltage V_(GS) of the cathoderegulation transistor device P_(CR) increasing based on a change in thevoltage across the resistor R₁. As a result of the increase of thegate-source voltage V_(GS) of the cathode regulation transistor deviceP_(CR), the activation of the cathode regulation transistor deviceP_(CR) (e.g., in the saturation mode) changes to decrease the amplitudeof the current I_(A) flowing through the cathode regulation transistordevice P_(CR) and to the gate of the power transistor N_(PWR). Inresponse, the decrease in the amplitude of the current I_(A) relative tothe current I_(C) results in a decrease of the gate voltage V_(G) of thepower transistor N_(PWR). Accordingly, the activation of the powertransistor N_(PWR) (e.g., in the saturation mode) changes to conduct alesser amplitude of the reverse current I_(REV), thereby decreasing thereverse voltage V_(CA) to approximately the predetermined thresholdamplitude V_(PT), and thus the Zener breakdown voltage of the Zenerdiode stack 208, in a reverse feedback manner.

In the example of FIG. 2, the reverse current controller 202 alsoincludes an indicator switch, demonstrated as an N-FET N_(IND) Theindicator switch N_(IND) has a gate coupled to the gate of the anoderegulation transistor device N_(AR), a source coupled to the firstmonitoring terminal 204, and a drain that can be coupled to a separateindication circuit (not shown) to provide a signal IND. Thus, theindicator switch N_(IND) is activated in response to the reverse voltageV_(CA) being approximately equal to the predetermined thresholdamplitude V_(PT) to indicate clamping of the reverse voltage V_(CA)(e.g., to monitoring circuitry that can be included in the associated ICor an external circuit). The signal IND can therefore provide indicationof the clamping of the reverse voltage V_(CA) to other circuitry, suchas to post fault information or to enable additional controls.

As described above, in response to changes to an increase in theamplitude of the voltage V_(C) or a decrease in the amplitude of thevoltage V_(A), the cathode regulation transistor device P_(CR) and theanode regulation transistor device N_(AR) can adjust the control of thepower transistor N_(PWR) to adjust the amplitude of the reverse currentI_(REV)to maintain clamping of the voltage V_(CA) at the predeterminedthreshold amplitude V_(PT). Accordingly, the reverse current I_(REV) canbe maintained at an amplitude that is sufficiently low to substantiallymitigate damage to the power transistor N_(PWR) and/or other circuitcomponents in the input stage 102. Also, the reverse current controller202 operates based on the voltages V_(A) and V_(C), with no need for anyadditional power or bias signals provided.

FIG. 3 is an example block diagram 300 of a reverse current controller302. The diagram 300 also demonstrates a power transistor, demonstratedin the example of FIG. 3 as an N-FET N_(PWR). The reverse currentcontroller 302 and the power transistor N_(PWR) can be the reversecurrent controller 114 and the power transistor 106, respectively, inthe example of FIG. 1. Therefore, reference is to be made to the exampleof FIG. 1 in the following description of the example of FIG. 3.

The reverse current controller 302 is arranged similar to the reversecurrent controller 202 in the example of FIG. 2. In the example of FIG.3, the reverse current controller 302 is coupled to a first monitoringterminal 304 and a second monitoring terminal 306. The power transistorN_(PWR) is arranged to have a source coupled the first monitoringterminal 304, having an anode voltage V_(A), and a drain coupled to thesecond monitoring terminal 306 to correspond, respectively, to an anodeand a cathode, having a cathode voltage V_(C), of an ideal diode formedby the power transistor N_(PWR) and the power switch control system 112.A control signal CTRL representative of an activation signal providedfrom control circuitry (not shown) is provided to a gate of the powertransistor N_(PWR) to control a voltage V_(G) at the gate of the powertransistor N_(PWR) for operating the power transistor N_(PWR) duringnormal operation of the power supply system 100.

In the example of FIG. 3, the reverse current controller 302 includes aresistor R₁ coupled to the second monitoring terminal 306 and a firstZener diode stack 308 that is arranged between the first monitoringterminal 304 and the resistor R₁. As an example, the first Zener diodestack 308 includes a quantity of Zener diodes that can, combined withthe resistor R₁, set a desired first predetermined threshold amplitudeV_(PT1) to which the reverse voltage V_(CA) is clamped. The reversecurrent controller 302 also includes a cathode regulation transistordevice P_(CR) having a source coupled to the second monitoring terminal306 and a gate coupled between the resistor R₁ and the first Zener diodestack 308. The drain of the cathode regulation transistor device P_(CR)is coupled to the gate via a Miller capacitor C_(M), and is coupled to agate of the power transistor N_(PWR) via a diode D₁. The reverse currentcontroller 302 also includes an anode regulation transistor deviceN_(AR), having a source coupled to the first monitoring terminal 304.The drain of the anode regulation transistor device N_(AR) is coupled tothe gate of the power transistor N_(PWR) via a resistor R₂.

In the example of FIG. 3, the reverse current controller 302 alsoincludes a resistor R₃ coupled to the second monitoring terminal 306 anda second Zener diode stack 312 that is arranged between the firstmonitoring terminal 304 and the resistor R₃. As an example, the secondZener diode stack 312 includes a quantity of Zener diodes that can,combined with the resistor R₃, set a desired second predeterminedthreshold amplitude V_(PT2) that is less than the first predeterminedthreshold amplitude V_(PT1). For example, the second Zener diode stack312 can include one fewer Zener diode than the first Zener diode stack308. In the example of FIG. 3, the gate of the anode regulationtransistor device N_(AR) is coupled between two of the Zener diodes inthe second Zener diode stack 312 (e.g., a first and a second Zener diodefrom the first monitoring terminal 304). The reverse current controller302 also includes an indicator switch, demonstrated as an N-FET N_(IND).The indicator switch N_(IND) has a gate coupled to the gate of the anoderegulation transistor device N_(AR), a source coupled to the firstmonitoring terminal 304, and a drain that can be coupled to a separateindication circuit (not shown) to provide a signal IND.

The reverse current controller 302 also includes a safety circuit 314that is coupled between the first and second Zener diode stacks 308 and312. The safety circuit 314 is configured to control a relative time ofactivation of the anode regulation transistor device N_(AR) and thecathode regulation transistor device P_(CR). As an example, the cathoderegulation transistor device P_(CR) can exhibit spurious activation inresponse to a transient in the voltage V_(C) based on the gate of thecathode regulation transistor device PC_(R) not tracking the voltageV_(C). The arrangement of the capacitor C_(M) between the gate and thedrain of the cathode regulation transistor device P_(CR) can result inactivation of the cathode regulation transistor device P_(CR), even ifthe drain-source voltage of the cathode regulation transistor deviceP_(CR) is less than the threshold. Thus, as described herein, the safetycircuit 314 can hold the cathode regulation transistor device P_(CR) ina deactivated state until the reverse voltage V_(CA) is approximatelyequal to the second predetermined threshold amplitude V_(PT2).

In the example of FIG. 3, the safety circuit 314 includes a first P-FETP₁ and a second P-FET P₂ that are arranged as diode-connected (e.g.,with drain and gate coupled to each other). The diode-connected P-FETsP₁ and P₂ provide for a third predetermined threshold amplitude V_(PT3)that is approximately two diode-drops in voltage from the voltage V_(C).The first P-FET P₁ is coupled at the source to the second monitoringterminal 306 and the second P-FET P₂ is coupled at the source to thedrain and gate of the first P-FET P₁. The second P-FET P₂ is coupled atthe drain and gate to a parallel connection of a resistor R₄ and acapacitor C1 that are each also coupled to the first monitoring terminal304. The safety circuit 314 also includes a third P-FET P3 that iscoupled at a source to the second monitoring terminal 306 and at a drainto the gate and drain of the second P-FET P₂. The third P-FET P₃ has agate that is coupled between the resistor R₃ and the second Zener diodestack 312. The safety circuit 314 further includes a fourth P-FET P₄that that is coupled at a source to the second monitoring terminal 306and at a drain to the gate of the cathode regulation transistor deviceP_(CR). The gate of the fourth P-FET P₄ is coupled to the gate and drainof the second P-FET P₂.

Similar to as described above in the example of FIG. 2, the powertransistor N_(PWR) is controlled by the control signal CTRL (e.g., fromcontrol circuitry (not shown) in the power switch control system 112)during normal operation of the power supply system 100. However, inresponse to the reverse voltage V_(CA) being approximately equal to thethird predetermined threshold amplitude V_(PT3) (e.g., approximately twodiode-drops in voltage from the voltage V_(C) as set by the P-FETs P₁and P₂ in the safety circuit 314) the safety circuit 314 activates theP-FET P₄ to hold the cathode regulation transistor device P_(CR) in adeactivated state. For example, the amplitude of the third predeterminedamplitude V_(PT3) across the P-FETs P₁ and P₂ relative to the voltageV_(C) is sufficient to activate the P-FET P₄, thus pulling the gate ofthe cathode regulation transistor device P_(CR) to be approximatelyequal to the voltage V_(C). Therefore, the safety circuit 314 canmitigate spurious activation of the cathode regulation transistor deviceP_(CR) from a transient in the voltage V_(C) during the reverse voltageevent.

In response to the reverse voltage V_(CA) being approximately equal tothe second predetermined threshold amplitude V_(PT2), such as set by thesecond Zener diode stack 312, a current I_(Z1) flows from the secondmonitoring terminal 306 to the first monitoring terminal 304 through theresistor R₃ and the second Zener diode stack 312. The current I_(Z1)thus provides a gate-source voltage for the anode regulation transistordevice N_(AR) and the P-FET P₃, thus sufficiently activating each of theanode regulation transistor device N_(AR) and the P-FET P₃. Theactivation of the P-FET P₃ pulls up the gate voltage of the P-FET P₄ toapproximately the voltage V_(C), thereby deactivating the P-FET P₄. As aresult, the safety circuit 314 is disabled, and no longer holds thecathode regulation transistor device P_(CR) in the deactivated state.The activation of the anode regulation transistor device N_(AR) providesa current I_(A) from the gate of the power transistor N_(PWR) throughthe resistor R₂ and through the anode regulation transistor deviceP_(CR) to the first monitoring terminal 304. The amplitude of thecurrent I_(A) therefore holds the power transistor N_(PWR) in thedeactivated state when the reverse voltage V_(CA) is greater than thesecond predetermined threshold amplitude V_(PT2) and less than the firstpredetermined threshold amplitude V_(PT1).

In response to the reverse voltage V_(CA) being approximately equal tothe first predetermined threshold amplitude V_(PT1), and thusapproximately equal to the breakdown voltage of the Zener diode stack308, a current I_(Z2) flows from the second monitoring terminal 306 tothe first monitoring terminal 304 through the resistor R₁ and the Zenerdiode stack 308. The current I_(Z2) thus provides a gate-source voltagefor the cathode regulation transistor device P_(CR) which is no longerheld in a deactivated state by the safety circuit 314, thus sufficientlyactivating the cathode regulation transistor device P_(CR). Theactivation of the cathode regulation transistor device P_(CR) provides acurrent I_(C) from the second monitoring terminal 306 through thecathode regulation transistor device P_(CR) to the gate of the powertransistor N_(PWR) via the diode D₁. The amplitudes of the currentsI_(A) and I_(C) can provide a sufficient amplitude of the voltage V_(G)to hold the power transistor N_(PWR) in an activated state, therebyconducting the reverse current I_(REV) that flows from the secondmonitoring terminal 306 to the first monitoring terminal 304, similar toas described above in the example of FIG. 2.

The reverse current controller 302 can thus clamp the voltage V_(CA) tothe first predetermined threshold amplitude V_(PT1), even with transientchanges to the voltages V_(A) and V_(C) (e.g., as modeled by the voltageV_(ISO) from a voltage source 310 through a resistor R_(ISO)) in areverse feedback manner as described above in the example of FIG. 2.However, by implementing the second Zener diode stack 312 and the safetycircuit 314, the operation of the reverse current controller 302 canprovide for sequential operation of the anode regulation transistordevice N_(AR) and the cathode regulation transistor device P_(CR) tomitigate spurious activation of the cathode regulation transistor deviceP_(CR) in response to transient changes to the voltage V_(C).

In this description, the term “couple” may cover connections,communications, or signal paths that enable a functional relationshipconsistent with this description. For example, if device A generates asignal to control device B to perform an action, then: (a) in a firstexample, device A is directly coupled to device B; or (b) in a secondexample, device A is indirectly coupled to device B through interveningcomponent C if intervening component C does not substantially alter thefunctional relationship between device A and device B, so device B iscontrolled by device A via the control signal generated by device A.

Also, in this description, a device that is “configured to” perform atask or function may be configured (e.g., programmed and/or hardwired)at a time of manufacturing by a manufacturer to perform the functionand/or may be configurable (or reconfigurable) by a user aftermanufacturing to perform the function and/or other additional oralternative functions. The configuring may be through firmware and/orsoftware programming of the device, through a construction and/or layoutof hardware components and interconnections of the device, or acombination thereof. Furthermore, a circuit or device described hereinas including certain components may instead be configured to couple tothose components to form the described circuitry or device. For example,a structure described as including one or more semiconductor elements(such as transistors), one or more passive elements (such as resistors,capacitors, and/or inductors), and/or one or more sources (such asvoltage and/or current sources) may instead include only thesemiconductor elements within a single physical device (e.g., asemiconductor wafer and/or integrated circuit (IC) package) and may beconfigured to couple to at least some of the passive elements and/or thesources to form the described structure, either at a time of manufactureor after a time of manufacture, such as by an end user and/or a thirdparty.

Modifications are possible in the described embodiments, and otherembodiments are possible, within the scope of the claims.

What is claimed is:
 1. A power switch control system comprising areverse current controller, the reverse current controller having afirst monitoring terminal, a second monitoring terminal, and a controloutput, the control output adapted to be coupled to a control input of apower transistor, the first and second monitoring terminals adapted tobe coupled to respective first and second terminals of the powertransistor so the power transistor and the power switch control systemare configured as an ideal diode between the first monitoring terminaland the second monitoring terminal, the reverse current controllerconfigured to provide a control signal at the control output to activatethe power transistor to conduct a reverse current from the secondmonitoring terminal to the first monitoring terminal responsive to areverse voltage associated with a voltage at the second monitoringterminal relative to a voltage at the first monitoring terminalexceeding a particular threshold amplitude.
 2. The system of claim 1,wherein the reverse current controller is configured to clamp thereverse voltage to the particular threshold amplitude to conduct thereverse current in response to the reverse voltage being approximatelyequal to the particular threshold amplitude.
 3. The system of claim 2,wherein the reverse current controller comprises: an anode regulationtransistor device coupled between an input of the power transistor andthe first monitoring terminal, the anode regulation transistor devicebeing activated in response to the reverse voltage being approximatelyequal to the particular threshold amplitude to control activation of thepower transistor; a cathode regulation transistor device coupled betweenthe input of the power transistor and the second monitoring terminal,the cathode regulation transistor device being activated in response tothe reverse voltage being approximately equal to the particularthreshold amplitude to control activation of the power transistor; and aZener diode stack coupled to an input of the anode regulation transistordevice and an input of the cathode regulation transistor device, theZener diode stack being configured to set the particular thresholdamplitude and to activate the anode regulation transistor device and thecathode regulation transistor device in response to the reverse voltagebeing approximately equal to the particular threshold amplitude.
 4. Thesystem of claim 3, wherein the reverse current controller comprises anindicator switch that is coupled to the Zener diode stack, so theindicator switch is activated in response to the reverse voltage beingapproximately equal to the particular threshold amplitude to indicateclamping of the reverse voltage.
 5. The system of claim 3, wherein thereverse current is a first reverse current, wherein the anode regulationtransistor device and the cathode regulation transistor device areactivated by the Zener diode stack in response to the reverse voltagebeing approximately equal to the particular threshold amplitude toconduct a second reverse current from the second monitoring terminal tothe first monitoring terminal, the second reverse current providing anactivation voltage for the power transistor to conduct the first reversecurrent through the power transistor.
 6. The system of claim 5, whereinthe first reverse current is controlled by the power transistor and thesecond reverse current is controlled by the anode regulation transistordevice and the cathode regulation transistor device to clamp the reversevoltage at approximately the particular threshold amplitude.
 7. Thesystem of claim 3, wherein the Zener diode stack is a first Zener diodestack configured to set the particular threshold amplitude as a firstthreshold amplitude, the system further comprising: a second Zener diodestack arranged in parallel with the first Zener diode stack, the secondZener diode stack being configured to set a second threshold amplitude,the second threshold amplitude being less than the first thresholdamplitude; and a safety circuit coupled to the second Zener diode stack,the safety circuit being configured to control a relative time ofactivation associated with the anode regulation transistor device andthe cathode regulation transistor device in response to the reversevoltage being approximately equal to the second threshold amplitude. 8.The system of claim 7, wherein the safety circuit comprises at least onediode configured to set a third threshold amplitude that is less thanthe second threshold amplitude, wherein the safety circuit is configuredto hold the anode regulation transistor device in a deactivated state inresponse to the reverse voltage being approximately equal to the thirdthreshold amplitude and less than the second threshold amplitude,wherein the safety circuit is configured to activate the anoderegulation transistor device in response to the reverse voltage beingapproximately equal to the second threshold amplitude.
 9. The system ofclaim 7, wherein the reverse current controller comprises an indicatorswitch that is coupled to the second Zener diode stack, so the indicatorswitch is activated in response to the reverse voltage beingapproximately equal to the particular threshold amplitude to indicateclamping of the reverse voltage.
 10. A power supply system comprising:an input stage configured to provide a power current in response to aninput voltage; an output stage configured to provide the power currentto a load; a power transistor arranged between the input stage and theoutput stage, the power transistor being activated to conduct the powercurrent from the input stage to the output stage; a power switch controlsystem configured to control the power transistor, the power switchcontrol system comprising: a first monitoring terminal coupled to afirst terminal of the power transistor; a second monitoring terminalcoupled to a second terminal of the power transistor, so the powertransistor and the power switch control system form an ideal diodebetween the first monitoring terminal arranged as an anode of the idealdiode and the second monitoring terminal arranged as a cathode of theideal diode; and a reverse current controller coupled to the firstmonitoring terminal and the second monitoring terminal and beingconfigured to control activation of the power transistor to conduct areverse current from the second monitoring terminal to the firstmonitoring terminal in response to a reverse voltage arranged as acathode voltage at the second monitoring terminal being greater than ananode voltage at the first monitoring terminal.
 11. The system of claim10, wherein the reverse current controller is configured to clamp thereverse voltage to a particular threshold amplitude to conduct thereverse current in response to the reverse voltage being approximatelyequal to the particular threshold amplitude.
 12. The system of claim 11,wherein the reverse current controller comprises: an anode regulationtransistor device coupled between an input of the power transistor andthe first monitoring terminal, the anode regulation transistor devicebeing activated in response to the reverse voltage being approximatelyequal to the particular threshold amplitude to control activation of thepower transistor; a cathode regulation transistor device coupled betweenthe input of the power transistor and the second monitoring terminal,the cathode regulation transistor device being activated in response tothe reverse voltage being approximately equal to the particularthreshold amplitude to control activation of the power transistor; and aZener diode stack coupled to an input of the anode regulation transistordevice and an input of the cathode regulation transistor device, theZener diode stack being configured to set the particular thresholdamplitude and to activate the anode regulation transistor device and thecathode regulation transistor device in response to the reverse voltagebeing approximately equal to the particular threshold amplitude.
 13. Thesystem of claim 12, wherein the reverse current is a first reversecurrent, wherein the anode regulation transistor device and the cathoderegulation transistor device are activated by the Zener diode stack inresponse to the reverse voltage being approximately equal to theparticular threshold amplitude to conduct a second reverse current fromthe second monitoring terminal to the first monitoring terminal, thesecond reverse current providing an activation voltage for the powertransistor to conduct the first reverse current through the powertransistor.
 14. The system of claim 13, wherein the first reversecurrent is controlled by the power transistor and the second reversecurrent is controlled by the anode regulation transistor device and thecathode regulation transistor device to clamp the reverse voltage atapproximately the particular threshold amplitude.
 15. The system ofclaim 12, wherein the Zener diode stack is a first Zener diode stackconfigured to set the particular threshold amplitude as a firstthreshold amplitude, the system further comprising: a second Zener diodestack arranged in parallel with the first Zener diode stack, the secondZener diode stack being configured to set a second threshold amplitude,the second threshold amplitude being less than the first thresholdamplitude; and a safety circuit coupled to the second Zener diode stack,the safety circuit being configured to control a relative time ofactivation associated with the anode regulation transistor device andthe cathode regulation transistor device in response to the reversevoltage being approximately equal to the second threshold amplitude. 16.The system of claim 15, wherein the safety circuit comprises at leastone diode configured to set a third threshold amplitude that is lessthan the second threshold amplitude, wherein the safety circuit isconfigured to hold the anode regulation transistor device in adeactivated state in response to the reverse voltage being approximatelyequal to the third threshold amplitude and less than the secondthreshold amplitude, wherein the safety circuit is configured toactivate the anode regulation transistor device in response to thereverse voltage being approximately equal to the second thresholdamplitude.
 17. An integrated circuit (IC) comprising: a Zener diodestack having an input coupled to a first monitoring terminal and anoutput coupled to a second monitoring terminal, the first and secondmonitoring terminals being coupled to receive a first terminal of apower transistor and a second terminal of the power transistor,respectively; an anode regulation transistor device having an inputcoupled to the Zener diode stack, having a first terminal coupled to thefirst monitoring terminal and a second terminal coupled to receive aninput of the power transistor; and a cathode regulation transistordevice having an input coupled to the Zener diode stack, having a firstterminal coupled to receive the input of the power transistor and asecond terminal coupled to the second monitoring terminal.
 18. The IC ofclaim 17, wherein the power transistor is configured to conduct a firstreverse current from the first monitoring terminal to the secondmonitoring terminal in response to a reverse voltage arranged as acathode voltage at the second monitoring terminal being greater than ananode voltage at the first monitoring terminal, wherein the anoderegulation transistor device and the cathode regulation transistordevice are activated by the Zener diode stack in response to the reversevoltage being approximately equal to a particular threshold amplitudeset by the Zener diode stack to conduct a second reverse current fromthe second monitoring terminal to the first monitoring terminal, thesecond reverse current providing an activation voltage for the powertransistor to conduct the first reverse current through the powertransistor.
 19. The IC of claim 18, wherein the first reverse current iscontrolled by the power transistor and the second reverse current iscontrolled by the anode regulation transistor device and the cathoderegulation transistor device to clamp the reverse voltage atapproximately the particular threshold amplitude.
 20. The IC of claim18, wherein the Zener diode stack is a first Zener diode stackconfigured to set the particular threshold amplitude as a firstthreshold amplitude, the system further comprising: a second Zener diodestack arranged in parallel with the first Zener diode stack, the secondZener diode stack being configured to set a second threshold amplitude,the second threshold amplitude being less than the first thresholdamplitude; and a safety circuit coupled to the second Zener diode stack,the safety circuit being configured to control a relative time ofactivation associated with the anode regulation transistor device andthe cathode regulation transistor device in response to the reversevoltage being approximately equal to the second threshold amplitude.